At more temperate latitudes and at lower altitudes, the ericaceous-dominated plant communities give way to forest ecosystems. In these coniferous, deciduous and mixed forest biomes, the array of plant litter chemistry is diverse, with a mixture of readily degradable and recalcitrant materials. In these ecosystems, ectomycorrhizae dominate as soils develop a "mor" and "moder" type of humus over more base-rich parent material. In these ecosystems phosphorus as well as nitrogen can be limiting to plant growth. Again, the ability of mycorrhizal fungi to behave as saprotrophs to effect a "direct cycling" of nutrients from partially decomposed organic residues is a benefit to the plant community. In this context the ability of ectomycorrhize to produce a range of enzymes is a benefit, allowing the host plant to obtain both nitrogen and phosphate from organic resources and to compete against immobilization by the saprotrophic soil microbial community (Dighton, 1991). Ectomycorrhizae have been shown to produce nitrogen-degrading protease enzymes (Abuzinadah and Read, 1986a, b; 1989; Read et al., 1989; Leake and Read, 1990a, b; Zhu et al., 1994; Tibbett et al., 1999; Anderson et al., 2001), phosphate-solubilizing acid phosphatase enzymes (Bartlett and Lewis, 1973; Dighton, 1983; Antibus et al., 1992; 1997; Leake and Miles, 1996; Joner and Johansen, 2000), and other enzymes (Giltrap, 1982; Durall et al., 1994), enabling them to utilize forest floor carbon. Dighton (1991) has reviewed the abilities of mycorrhizal plants to utilize organic nutrients.
Abuzinadah and Read (1986a, b) demonstrated the use of peptides and proteins as nitrogen sources by ectomycorrhizae in culture and in symbiosis. Four tree species in mycorrhizal association with the fungus Hebeloma crustuliniforme were shown to be able to incorporate up to 53% of the total N contained in proteins or peptides, whereas nonmycorrhizal tree seedlings could access no nitrogen from these organic sources. Similarly, Wallander et al. (1997) showed that the uptake of nitrogen from alanine or ammonium was 10 times higher than from nitrate sources. In forested ecosystems, in which the decomposition rate and mineralization of nitrogen from plant residues is reduced because of low resource the quality (high C:N ratio) we have seen that heterotrophic microbial communities are capable of importing nitrogen (or other nutrients) from the surrounding environment (deeper soil horizons, patches of high rates of mineralization) into the low-quality resource in order to effect more rapid decomposition by lowering the C:N ratio. Similarly, in arctic regions, in which decomposition and nutrient mineralization is constrained by low temperatures, Tibbett et al. (1998a) suggest that there has been a pre adaptation of Hebeloma u
species to utilize nitrogen in the form of proteins and glutamic acid, which are often released from organic matter during freezing. Indeed, they (Tibbett et al., 1998b, c) demonstrate that cold active phosphomonoesterase enzyme is only produced by Hebeloma when grown at 6°C. There is thus competition between the saprotrophic and mycorrhizal fungi for readily available nutrients (Kaye and Hart, 1997). This is particularly true if there is an abundance of nitrifying bacteria in the system, which utilize NH4 + as an energy source rather than carbon (Tate, 1995). These nitrifiers can consume considerable quantities of NH4 (possibly up to 70% of the total available NH4) and be in direct competition with plant roots and their mycorrhizae (Norton and Firestone, 1996; Kaye and Hart, 1997). Although Yamanaka (1999) showed that the ectomycorrhizal fungi Laccaria bicolorcould utilize ammonium, nitrate, and urea as sources of nitrogen and Hebeloma spp. could also use bovine serum albumin, none of the mycorrhizal fungi could utilize nitrogen in the form of ethylenediamine or putrescine, suggesting that the ectomycorrhizal fungi could not compete with saprotrophic fungi for resources in decaying animal carcasses.
Bartlett and Lewis (1973) demonstrated the production of surface acid phosphatases by beech mycorrhizae and suggested their potential importance for phosphate acquisition by ectomycorrhizal plants from both complex inorganic and organic forms of phosphorus in the soil. As Haussling and Marschner (1989) determined that approximately 50% of the phosphorus in a Norway spruce forest was in the form of organic P, the benefit of the ability of ectomycorrhizal-associated forest trees to produce phosphatase enzymes was evident. They demonstrated that there was a two- to 2.5-fold increase in acid phosphatase activity in the rhizosphere as compared to the bulk soil. The ability to ectomycorrhizal fungi to access and incorporate phosphorus from complex organic forms of P, such as inositol hexaphosphate, has been demonstrated a number of times (Dighton, 1983; Mousain and Salsac, 1986; Antibus et al., 1992; 1997) and the regulation of the expression of this enzyme by external concentrations of orthophosphate has been shown by MacFall et al. (1991). Indeed, Antibus et al. (1992) showed that in some ectomycorrhizal fungi there was a greater uptake of phosphorus from organic supplies than from inorganic supplies because of the action of acid phosphatase and phytase enzymes (Table 3.11). In a mixed forest ecosystem, the benefit of ectomycorrhizal associations with tree species is shown to be an advantage in terms of the accession of P from both inorganic and organic sources, compared to an arbuscular mycorrhizal tree species (Antibus et al., 1997) (Fig. 3.9). In addition, there is evidence to show that ectomycorrhizae are able to access phosphorus from complex inorganic forms of phosphate (Lapeyrie et al., 1991). Paxillus involutus was able to solubilize calcium phosphate, but only in the presence of available ammonium or nitrate nitrogen. Other fungal species examined, however, could only solubilize this form of phosphate in the presence of u
Table 3.11 Incorporation of 32P Labeled Phosphorus (CPM mgdm_1h_1) into Ectomycorrhizal Fungal Mycelia from Either Inorganic (Pj) and Organic (Po) Sources Due to the Activity of Mycelial Surface or Soluble Acid Phosphatase (pNPPase-pNPP release mgdm_1h_1) or Phytase (nmol P Released mg/(proteinh) Enzyme Activity pNPPase Phytase P --- --- 32P
Fungal species source Mycelium Soluble Mycelium Soluble uptake
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